We strived to make sure that everything you need to establish a
VPN using Linux was contained in Building Linux Virtual Private Networks. However these are some
other books we'd suggest if you are looking for additional information
to help with Linux and security related issues.

Your VPN is only as secure as your Linux machines.
Hacking Linux Exposed, Second Edition (by one of the Building Linux Virtual Private Networks
authors) shows you everything you need to keep your machine safe from intruders.

Step-by-step descriptions of how to configure Linux firewalls, both ipchains and iptables. This book will help you limit the access of your VPN users if you want additional security. It doesn't specifically cover VPN and firewall issues, but we cover those in Building Linux Virtual Private Networks already.